The present invention relates to the processing of wood pulp, and particularly to improvement in the bleaching of wood pulp by hydrogen peroxide.
It is common practice in the pulp and paper industry to treat wood pulp to improve its color and brightness. To a large extent, pulp color and brightness determines its usefulness and value. Considerations of visual appeal now apply not only to fine paper and printing products of the pulp and paper industry, but extend to packaging papers and boards, newsprint, and the like.
In the bleaching process, the pulp is contacted with any of several common bleaching agents, for example chlorine dioxide, a hypochlorite compound, or hydrogen peroxide.
The present invention particularly relates to an improvement in the bleaching of pulp with hydrogen peroxide. Although the potential for use of hydrogen peroxide in wood pulp bleaching is well known in the art, hydrogen peroxide bleaching has in the past found relatively limited use in actual practice. Today, however, interest in hydrogen peroxide wood pulp bleaching is increasing for a variety of reasons. For instance, there is increased emphasis today in the processing of pulp by thermal and mechanical rather than chemical means. Thermal and mechanical processing often provides economies in operation and a stronger paper product. However, thermal and mechanical processing yields a product which has a higher content of lignin, unsaturated fatty material, and other substances responsible for color formation in paper products prepared from the pulp. As a result, a more severe bleaching is required of the thermal/mechanical pulp than of a corresponding chemical pulp to achieve the desired color and brightness. Under these conditions hydrogen peroxide is generally effective in significantly lower quantity, compared to the use of chlorine bleaches.
There is now also an increased interest in the production of alkaline papers, for reasons relating to convenience and cost in the paper making process and to strength and life of the finished paper. Hydrogen peroxide is known to be very useful and effective under alkaline conditions.
It has further been reported that hydrogen peroxide improves ink particle dispersion when bleaching takes place in combination with the deinking of printed papers, reduces brightness reversion in paper products prepared from bleached pulp, enhances drainage characteristics of the pulp fibers during papermaking, and attacks cellulose fibers to a lesser extent than do chlorine bleaches.
The object of this invention is an improvement in the performance of processes for the hydrogen peroxide bleaching of wood pulp. In one important aspect, the present invention is directed to a process in which an alkenylsuccinic anhydride compound is incorporated into the aqueous pulp bleaching medium as a promoter for the bleaching activity. It is known in the art, from the disclosure of the published German patent application No. 3,247,061, to incorporate into a peroxide-containing bath for the bleaching of cellulose fiber materials (e.g., cotton) a mixture of phosphoric acid with an alkylcarboxylic anhydride in which the alkyl group has 2 to 4 carbon atoms and is preferably acetic anhydride. Netherlands patent application No. 7109629 has been reported to describe a powdered laundry washing composition containing anionic, nonionic or soap washing agents, builders, an oxygen-liberating bleaching agent such as sodium perborate, and an alkenyl- or alkylsuccinic anhydride bleach activator wherein the alkenyl or alkyl group has from 6 to 12 carbon atoms. Japanese patent application No. 57029662, discloses the hydrogen peroxide bleaching of cloth or yarn at low pH (e.g., 4 to 7) and at high temperature (e.g., 50.degree. to 120.degree. C.) in the presence of an acid anhydride such as acetic anhydride, succinic anhydride, or phthalic anhydride. It is further in the art (i.e., from disclosures in German patent application No. 3438529 and European patent application No. 179223) that alkyl- and aryl-substituted diperoxysuccinic acids, having utility as bleaches and bleach boosters, are prepared by reacting a 2-alkyl- or aryl-substituted succinic anhydride, wherein the alkyl or aryl group has from 4 to 20 carbon atoms, with a mixture of hydrogen peroxide, water, and sulfuric acid.